FIGS. 1 and 2 illustrate a conventional matrixed display device which provides a monochrome display in response to an input video signal. The display device comprises first and second arrays of elongate, strip-form electrodes 2 and 4 supported on respective transparent substrates 6 and 8. Generally, the two arrays of electrodes are oriented orthogonally to one another. When the display device is positioned for normal use with the substrates vertical, the electrodes of the first array (referred to as the horizontal electrodes) extend horizontally and are spaced apart in the vertical direction, and the electrodes of the second array (referred to as the vertical electrodes) extend vertically and are spaced apart in the horizontal direction. The two arrays of electrodes are in spaced, crossing relationship. The positions of the horizontal electrodes 2 are shown in dashed lines in FIG. 2. At each crossing point there is an optical body 10. The term "optical body" is used herein to denote a body which controls the passage of light towards a viewer of the display device in dependence upon the electrical condition of the body. An optical body might be, for example, a liquid crystal device (LCD) which allows or prevents passage of light from a light source towards the viewer in dependence upon the electric field existing across the LCD, or a volume of electroluminescent (EL) material which emits or does not emit light in dependence upon the electric excitation to the volume of EL material. The horizontal electrodes 2 are connected to a horizontal electrode driver 12 and the vertical electrodes are connected to a vertical electrode driver 14. The vertical and horizontal electrode drivers include decoding and amplifying circuits and receive input signals from a video processor 20. The video processor 20 has an input terminal 18 at which it receives an input video signal.
The input video signal that is applied to the terminal 18 is composed of a succession of line intervals separated from each other by a horizontal sync pulse. During the active interval T.sub.L of the horizontal line time, the video signal is applied to the vertical electrode drive 14 over a line 19. If, for example, there are 300 electrodes in the horizontal array, the video processor generates a dot clock signal having 300 pulses during the active interval T.sub.L of each line and applies this signal to the vertical electrode driver 14. Analog switches (not shown) are connected between the line 19 and the vertical electrodes 4 respectively. The dot clock signal causes the driver 14 to close (i.e. render conductive) the analog switches sequentially and accordingly the video signal on the line 19 is sampled and successive sample values are applied to the vertical electrodes respectively.
For each horizontal sync pulse in the active interval of each field of the video signal, the video processor generates a row clock pulse, and after a predetermined number of row clock pulses the video processor generates a vertical reset pulse. For example, if the display device has an aspect ratio of 1:1 and there are 300 horizontal electrodes, there would be a vertical reset pulse after 300 horizontal sync pulses. The row clock signal is applied to the horizontal electrode driver 12. The row clock signal is used by the driver 12 to enable it to select horizontal electrodes 2 sequentially. The driver 12 applies a negative potential to the selected electrode 2. In this manner, the optical bodies are addressed successively and the electrical condition of an optical body depends on the level of the video signal when the body is addressed.
In a monochrome matrixed display device, each optical body corresponds to a single, separately addressable pixel. The size of the optical body depends of the width W and spacing of the stripform electrodes 2 and 4. The reciprocal of the sum (W+S) is a measure of the resolution of the display.
A matrixed LC display device has been used in a flat panel color television set. In such a display device, an optical bandpass filter element (not shown) is associated with each optical body. The filters are formed by thin film deposition on the vertical electrodes 4. The filter elements are arranged in three distinct arrays in accordance with their pass bands. Thus, the pass bank of the filter elements of a first array is in the red region of the optical spectrum, that of the filter elements of a second array is in the green region, and that of the filter elements of the third array is in the blue region. A color pixel is composed of a group of three adjacent monochrome pixels having red, green and blue filter elements respectively. In the conventional arrangement of monochrome pixels and associated filter elements that is shown in FIG. 3, the three monochrome pixels that comprise a color pixel are arranged side by side in the horizontal direction and are addressed by selecting respective vertical electrodes 4 in conjunction with a single horizontal electrode 2. The video processor 20 of the color display device receives a color component video signal and includes a digitizer which generates three color component signals each having a stream of 100 digital words per line interval. The driver 14 determines which group of three vertical electrodes is to be addressed, and the three digital words associated with that group determine the potentials that will be applied to those electrodes respectively. The driver 12 determines which horizontal electrode is to be addressed.
It is generally considered desirable that the aspect ratio of a color pixel be substantially the same as the aspect ratio of the display device in which it appears. In the case of a display device having an aspect ratio of 1:1 therefore, the vertical dimension of a color pixel is about the same as its horizontal dimension. This implies that the horizontal electrodes of the color display device shown in FIG. 3 are about three times as wide as the vertical electrodes and that the number of horizontal electrodes is only 100 instead of 300; and that each monochrome pixel shown in FIG. 3 has the form of a vertically-elongated rectangle having an aspect ratio of about 1:3. The minimum linear dimension of a color pixel is therefore about three times the minimum linear dimension of a monochrome pixel. The resolution of the color display is only about one-third that of a monochrome display having electrodes with the same minimum width and spacing. In order to illuminate two adjacent color pixels in a given row, it is necessary to address six of the vertical electrodes 2.
In a display device in which each color pixel is composed of a triangular arrangement of monochrome pixels, similar to the arrangement of phosphor deposits in a delta gun color CRT, the size of the color pixel can be reduced substantially compared with the arrangement shown in FIG. 3, if the vertical electrodes are composed of hourglass-shaped segments as shown in M. Suginoya et al, "Multicolor Graphic LCD with Tri-Colored Layers Formed by Electrodeposition," Proceedings, Third International Display Research Conference, 1983, pages 206-209. Electrodes of this configuration are more difficult to fabricate than uniform width electrodes and moreover the waisting of the electrodes increases the likelihood of an open circuit and, even if there are no open circuits, results in local regions of increased resistance and causes there to be a significant variation in the line time between the top row of pixels and the bottom row of pixels.
In a third conventional pattern, the sequence in which the filter elements occur in the horizontal direction changes from line to line down a column of color pixels. The pattern repeats every three horizontal lines. For a given width of monochrome pixel, the height of each monochrome pixel is the same as that in the FIG. 3 arrangement, and accordingly there is no improvement in resolution in the third arrangement. Moreover, the fact that the sequence of monochrome pixels in a color pixel varies in a column of color pixels from line to line to line introduces complexity into the addressing of the display device. The three filter patterns that are discussed above are also described in S. Tsuruta et al, "Color Pixel Arrangement Evaluation for LC-TV", Proceedings, 1985 International Display Research Conference, 1985, pages 24-26. Other articles relating to matrixed color display devices are T. Uchida et al, "A Full-Color Matrix LCD with Color Layers on the Electrodes", Proceedings, 1982 International Display Research Conference, 1982, pages 166-170; T. Uchida, "Color LCDs: Technological Developments", Proceedings, Third International Display Research Conference, 1983, section 5.1: and M. Sugata et al, "A TFT-Addressed Liquid Crystal Color Display", Proceedings, Third International Display Research Conference, 1983, pages 210-212.